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Welding Field Issues — Complete Troubleshooting Guide

Porosity

Types and Root Causes

Hydrogen porosity: Moisture in electrode coating, flux, base metal, shielding gas → H₂O decomposes in arc → atomic H dissolves in molten weld → as weld solidifies, H₂ solubility drops dramatically → gas bubbles trapped Most common: SMAW with wet/damaged electrodes; FCAW with damp flux core; GMAW with moisture in shielding gas

Nitrogen porosity: Air entrainment in shielding gas → N₂ dissolves in weld pool → bubbles on solidification Cause: shield gas flow too low, cross-wind, wrong cup size, contaminated gas line

CO porosity (carbon steel): High carbon + oxygen → CO gas at high temperature; reacts: FeO + C → Fe + CO Prevention: use deoxidizers (Si, Mn in electrode); insufficient deoxidation → CO porosity

Diagnosis by location: Surface porosity: gas not escaping; too fast travel speed or wrong shielding Subsurface (volumetric): hydrogen from moisture; measure by radiography or UT Wormhole porosity (elongated pores): nitrogen; air entrainment during solidification

Prevention/fix:

• Bake SMAW electrodes: E7018 → 300–370°C, 1 hour before use (ASME IX); store in rod oven at 120°C
• Preheat base metal to >10°C above dew point
• Shielding gas flow: 15–25 L/min GMAW; reduce if wind (use wind shield or increase flow rate)
• Clean base metal: remove mill scale, oil, paint within 25 mm of weld
• Repair: grind out pores to bright metal; re-weld with proper procedure

Hot Cracking (Solidification and Liquation)

Solidification Cracking

Mechanism: During solidification, liquid film trapped between dendrites; tensile strain (from shrinkage) exceeds ductility of semi-solid → crack along grain boundaries High risk: high sulfur/phosphorus (forms low-melting eutectics FeS, Fe₃P); high carbon; high C:Mn ratio

Carbon equivalent for hot cracking susceptibility: Ito-Bessyo: Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B UCS (Units of Cracking Susceptibility) = 230C + 190S + 75P + 45Nb − 12.3Si − 5.4Mn − 1 High susceptibility: UCS > 25; low: UCS < 10

Restrastraint test (Varestraint): Controlled bending strain applied during welding → measure total crack length (TCL); threshold augmented strain ε_th below which no cracking Used to rank consumable susceptibility; higher ε_th = more resistant

Field diagnosis: Centerline crack in weld (longitudinal); appears immediately on cooling; visible without NDT on surface; red-end-check test shows crack at high temperature

Solutions:

• Limit heat input: reduces segregation zone size
• Weld bead shape: width/depth ratio W:D > 1.0 (avoid deep narrow beads — worst for centerline cracking)
• Lower S+P in base metal; use low-S filler metal (< 0.008% S)
• Preheat for high-restraint joints (reduces shrinkage stress rate)
• Back-step technique to reduce end-crater cracking

Liquation Cracking (HAZ)

Mechanism: In HAZ, grain boundary liquation of low-melting phases (sulfides, carbides) → liquid films at grain boundaries → tensile stress from weld shrinkage → intergranular crack Most common in: nickel alloys (Inconel 600, 718 — worst), austenitic SS with niobium, some aluminum alloys

Field diagnosis: HAZ cracks, typically 1–3 mm from fusion line; found by PT or MT after cooling; intergranular fracture surface on SEM

Solutions:

• Reduce heat input (limits HAZ width and peak temperature)
• Solution anneal before welding to dissolve grain boundary phases
• For Inconel 718: use Inconel 625 filler (less susceptible); reduce welding current; use pulsed GTAW

Cold Cracking / Hydrogen-Induced Cracking (HIC)

Mechanism

Three conditions ALL required simultaneously:

1. Susceptible microstructure (martensite; high hardness > HRC 22)
2. Diffusible hydrogen in weld/HAZ (from moisture, contamination)
3. Tensile stress (residual + applied)

Remove any one → no cold crack. Occurs 48–72 hours after welding (delayed cracking).

Preheat calculation (ASME IX / CEN method): Carbon equivalent: CE_IIW = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 Minimum preheat temperature (AWS D1.1 prequalified): T_p [°C] = 350 × √(CE − 0.25) [Seferian formula; approximate] Or use ASME D-10.8 nomograph with CE, hydrogen level (H4/H8/H16), and restraint level

Hydrogen scale (EN ISO 3690): H4: < 4 mL H₂/100g deposited weld metal (low hydrogen SMAW, baked E7018) H8: 4–8 mL H₂/100g (standard) H16: 8–16 mL/100g (high — not acceptable for > 50 ksi yield)

Field preheat rules (carbon steel SMAW, medium restraint): CE < 0.40: no preheat (t < 20 mm) CE 0.40–0.50: 100°C preheat CE 0.50–0.60: 150–200°C preheat CE > 0.60: 200–300°C preheat

Diagnosis: Underbead cracks (HAZ, parallel to fusion line); toe cracks; root cracks; delayed appearance; transgranular or intergranular fracture; magnetic particle reveals HAZ cracks

Solutions:

• Low-hydrogen electrodes (E7018): baked to manufacturer spec; hermetic containers
• Preheat to calculated T_p; maintain interpass temperature (usually same as preheat min)
• Slow cool or PWHT immediately after welding (hydrogen bake-out: 200–300°C, 1h/25mm thickness)
• Post-weld hydrogen release: hold at 200°C for 4 hours before cool-down (if PWHT not done immediately)

Lamellar Tearing

Mechanism

Through-thickness tensile stress (from weld shrinkage perpendicular to plate surface) → tears along planar inclusions (MnS stringers from rolling) → step-shaped fracture below weld

Z-direction ductility (ASTM A770): Z-test: tensile specimen pulled through thickness; fracture area reduction (RA_Z) RA_Z ≥ 35%: low lamellar tearing risk (Z35 steel) RA_Z < 15%: high risk; do not use in T-joints with through-thickness stress

High-risk joints: T-joints, corner joints, set-on nozzles, orthogonal stiffeners — any joint pulling through the plate thickness

Solutions:

• Use Z35 or Z25 steel at critical joints (AS/EN 10164)
• Buttering: build up low-strength, high-ductility layer on plate surface before welding T-joint
• Shift fusion boundary: angle weld so it doesn't align with plate mid-thickness
• Replace T-joint with set-through nozzle design
• Use forged nozzle (no inclusions from rolling)

Distortion

Types and Prediction

Angular distortion (fillet welds): θ ≈ 0.02 × Q / (t × v) [degrees; Q = heat input [J/mm] = V×I/(v); t = plate thickness; v = travel speed] Transverse distortion: Δ_t ≈ K × A_weld / t [K ≈ 0.1–0.2 for butt welds; A_weld = weld cross section]

Root causes:

• Non-symmetric weld sequence
• Inadequate clamping during welding
• Wrong joint fit-up (excessive root gap → more weld metal → more shrinkage)
• High heat input

Control methods: Pre-setting (pre-cambering): plate bent opposite to expected distortion before welding Weld sequence: backstep; balanced welding (symmetric about neutral axis) Strongbacks and fixtures: mechanical restraint Thermal pre-tensioning: flame straightening post-weld Peening (only intermediate passes): compressive stress at weld toe Stress-relief (PWHT): doesn't remove distortion but prevents springback on cut

PWHT Field Issues

Common PWHT Problems

Insufficient temperature: ASME VIII UCS-56: min PWHT temp for CS = 593°C (1100°F) for t > 38 mm; 593°C for lower thicknesses (different per P-number) Below minimum → residual stresses not adequately relieved → SCC risk in service; HIC risk in wet H₂S Field failure: furnace thermocouple calibration error → actual temp 50°C below setpoint → PWHT credit invalid

Wrong heating rate: Max rate for PWHT: 200°C/h (above 315°C); or 5500/t [°C/h] (t = wall thickness in mm) per ASME Too fast → thermal gradients → thermal stress cracking (especially in thick-to-thin junctions)

Incorrect thermocouple placement: Thermocouples attached to surface (not buried in insulation) → surface reads correct but interior may not reach temperature ASME: thermocouples must be on or embedded in part; soak band ≥ 4× wall thickness each side of weld

Inadequate soaking time: ASME UCS-56: 1h/25mm minimum; not less than 1h Short hold → partial stress relief → high residual stress remains

Re-heat cracking (relaxation cracking): Cr-Mo steels (P91, P22) + PWHT above precipitation temperature → creep + simultaneous strengthening from carbide precipitation → intergranular cracking during PWHT Prevention: ensure PWHT temp is per WPS; avoid temp above upper transformation range for P91 (815°C max)

Sensitization of austenitic SS during PWHT: SS 304: sensitization at 450–870°C (chromium carbide precipitation at grain boundaries → Cr-depleted zone → intergranular corrosion) Never PWHT 304/316 in sensitization range; use stabilized grades (321, 347) or L grades (304L, 316L) if PWHT needed

Incomplete Fusion / Lack of Penetration

Incomplete fusion (IF): weld metal adjacent but not fused to base metal; planar discontinuity Causes: too-low heat input; incorrect torch angle; incorrect travel speed; wrong groove angle (< 60° for SMAW) Most serious discontinuity: planar → stress concentrator; acts like embedded crack

Incomplete penetration (IP): weld metal doesn't reach root of joint Causes: root gap too small; electrode too large for gap; burn-through risk if gap too large Single-sided pipe welds: target complete penetration; verify by back gouging + RT/UT

Undercut: groove melted into base metal at weld toe not filled by weld metal ASME B31.3: max undercut = 0.8 mm (1/32 in) for t ≤ 25 mm; not permitted at sharp notch angle Fatigue stress concentration at undercut → fatigue crack initiation → especially critical for cyclic loading

Dissimilar Metal Welding Issues

Carbon steel to stainless steel: Use Inconel 82/182 (ERNiCr-3) filler: prevents carbon migration (decarburization in CS, carburization in SS HAZ) Carbon migration: at service temperature 400–600°C → C migrates from CS to SS → CS side decarburized (weak) → creep failure Buttering: apply 309L SS or Inconel 625 to CS side, PWHT if required, then weld with 309L to SS

Martensitic SS to austenitic SS: Preheat martensitic side (410, 420) to 200–315°C; use austenitic filler (309L); slow cool; PWHT

P91 to P22 or P91 to 304SS: Critical: different PWHT requirements; PWHT P91 = 760°C; PWHT 304 → sensitization Solution: buttering on P91 side at P91 PWHT temp; then weld buttered surface to SS without PWHT

Arc Strikes

Arc strike: accidental arc contact outside weld zone → rapid local heat cycle → hard martensite → HIC nucleation site ASME B31.3: arc strikes not permitted in pressure-containing welds AWS D1.1: arc strikes not permitted; must be ground and MT/PT inspected; hardness check

Repair: Grind arc strike to bright metal; MT/PT to confirm no cracks; hardness check (< HRC 22 for CS); re-inspect

Standards

Output

Identify defect type: porosity / hot crack / cold crack / lamellar tearing / distortion / IF / IP / undercut / arc strike / PWHT issue. For each: Porosity: type (H₂/N₂/CO); root cause (moisture/shielding/base metal); size and distribution; acceptable per ASME/AWS? Hot crack: Pcm or UCS; weld bead W:D ratio; recommendation: low-heat-input, wider bead, low-S filler Cold crack: CE_IIW; diffusible H level (H4/H8/H16); T_preheat required [°C]; residual stress condition Lamellar tearing: RA_Z from mill cert [%]; joint type; buttering required? Distortion: measured angle [°]; predicted θ from Q/tv formula; mitigation: pre-camber or fixture PWHT: temp achieved vs. ASME minimum [°C]; rate [°C/h] vs. maximum; soak time [min/mm]; thermocouple placement Repair: grind-out method; re-weld: same or different WPS; re-inspect: MT/PT/UT/RT Applicable code: ASME IX / AWS D1.1 / API 582.